A review of mechanistic insight and application of pH-sensitive liposomes in drug delivery

Abstract

The pH-sensitive liposomes have been extensively used as an alternative to conventional liposomes in effective intracellular delivery of therapeutics/antigen/DNA/diagnostics to various compartments of the target cell. Such liposomes are destabilized under acidic conditions of the endocytotic pathway as they usually contain pH-sensitive lipid components. Therefore, the encapsulated content is delivered into the intracellular bio-environment through destabilization or its fusion with the endosomal membrane. The therapeutic efficacy of pH-sensitive liposomes enables them as biomaterial with commercial utility especially in cancer treatment. In addition, targeting ligands including antibodies can be anchored on the surface of pH-sensitive liposomes to target specific cell surface receptors/antigen present on tumor cells. These vesicles have also been widely explored for antigen delivery and serve as immunological adjuvant to enhance the immune response to antigens. The present review deals with recent research updates on application of pH-sensitive liposomes in chemotherapy/diagnostics/antigen/gene delivery etc.     

增强抗肿瘤药物肿瘤靶向性递送的叶酸修饰的pH敏感聚合物胶束（英文）

为增强抗肿瘤药物的聚合物胶束的肿瘤靶向性递送,本研究基于叶酸修饰的聚-(2-乙基-2-噁唑啉)-聚乳酸嵌段共聚物和聚-(2-乙基-2-噁唑啉)-聚乳酸嵌段共聚物,设计并制备了肿瘤细胞表面过表达的叶酸受体的配体叶酸修饰的pH敏感的聚合物胶束,用于包载抗肿瘤药物阿霉素。所制备的载阿霉素的聚合物胶束的粒径约为36nm,粒径分布较窄,透射电镜观察胶束呈规则的球形,并具有pH敏感的释药行为。此外,体外的细胞摄取、细胞毒性以及荷瘤裸鼠体内的近红外荧光成像的结果均证明了所制备的聚合物胶束增强的肿瘤靶向性,并归因于叶酸受体的介导和胶束pH敏感性的协同作用。因此,靶向叶酸受体的pH敏感胶束在肿瘤靶向递送难溶药物方面具有一定的潜力。     

Effective delivery of an angiogenesis inhibitor by neovessel-targeted liposomes

Angiogenesis is critical for tumor growth and metastasis, and several angiogenesis inhibitors have been developed for the treatment of cancer. Previously, we identified angiogenic vessel-homing peptide, Ala-Pro-Arg-Pro-Gly (APRPG), by use of a phage-displayed peptide library. APRPG peptide-modified liposomes have been revealed to be useful for the delivery of encapsulated drugs to angiogenic vasculature in tumor-bearing animals. In the present study, to assess the usefulness of APRPG-PEG-modified liposomes as a carrier of angiogenesis inhibitors in vitro and in vivo, we designed and validated APRPG-PEG-modified liposomal angiogenesis inhibitor. SU1498, an inhibitor of vascular endothelial growth factor (VEGF) receptor tyrosine kinase, was successfully encapsulated into the liposomes. APRPG-PEG-modified liposomal SU1498 inhibited VEGF-stimulated endothelial cell proliferation in vitro. Moreover, APRPG-PEG-modified liposomal SU1498 significantly decreased tumor microvessel density in Colon26 NL-17 cell-bearing mice and prolonged the survival time of the mice. These findings suggest that APRPG-PEG-modified liposomes effectively deliver SU1498 to angiogenic endothelial cells in tumors and thus inhibit tumor-induced angiogenesis.     

Chitosan-coated liposomes for intracellular oligonucleotides delivery: Characteristics and cell uptake behavior

Surface modification of liposomes with polymer to optimize drug delivery was well developed recently. The objective of the present work was to evaluate the feasibility of chitosan-coated liposomes (CSLP) as vehicles for anti-sense oligodeoxynucleotides (ASON). CSLP was obtained by adding chitosan dropwise to liposomes under magnetic stirring. The effect of chitosan content on size, zeta potential, and coating efficiency was investigated, which showed that chitosan increased the size and zeta potential of CSLP, and the coating efficiency increased with chitosan content increasing. Agarose gel electrophoresis was employed to evaluate the loading efficiency of CSLP for ASON, from which one could see ASON was completely combined to CSLP when the mass ratio of total lipids:ASON was more than 50:1. MTT assay showed that CSLP took on very low cytotoxicity, which is much lower than chitosan. At last, cell uptake behavior was investigated by a flow cytometer, which showed that CSLP enhanced significantly the COS7 cells uptake of ASON. All the results indicated that the CSLP could be a promising non-viral ASON vehicle.     

Long-circulating, pH-sensitive liposomes sterically stabilized by copolymers bearing short blocks of lipid-mimetic units

A major hurdle towards in vivo utilization of pH-sensitive liposomes is their prompt sequestration by reticuloendothelial system and hence short circulation time. Prolonged circulation of liposomes is usually achieved by incorporation of pegylated lipids, which have been frequently reported to deteriorate the acid-triggered release. In this study we evaluate the ability of four novel nonionic copolymers, bearing short blocks of lipid-mimetic units to provide steric stabilization of DOPE:CHEMs liposomes. The vesicles were prepared using the lipid film hydration method and extrusion, yielding liposomes of 120–160 nm in size. Their pH-sensitivity was monitored via the release of encapsulated calcein. The incorporation of the block copolymers at concentration up to 10 mol% did not deteriorate the pH-sensitivity of the liposomes. A selected formulation was tested for stability in presence of 25% human plasma and proved to significantly outclass the plain DOPE:CHEMs liposomes. The ability of calcein-loaded liposomes to deliver their cargo inside EJ cells was investigated using fluorescent microscopy and the results show that the surface-modified vesicles are as effective to ensure intracellular delivery as plain liposomes. The pharmacokinetics and organ distribution of a selected formulation, containing a copolymer bearing four lipid anchors was investigated in comparison to plain liposomes and PEG (2000)–DSPE stabilized liposomes. The juxtaposition of the blood clearance curves and the calculated pharmacokinetic parameters show that the block copolymer confers superior longevity in vivo. The block copolymers utilized in this study can be consider as promising sterically stabilizing agents for pH-sensitive liposomes.     

Multivesicular liposomes for oral delivery of recombinant human epidermal growth factor

The purpose of the present study was to prepare multivesicular liposomes with a high drug loading capacity and to investigate its potential applicability in the oral delivery of a peptide, human epidermal growth factor (rhEGF). The multivesicular liposomes containing rhEGF was prepared by a two-step water-in-oil-in-water double emulsification process. The loading efficiency was increased as rhEGF concentration increased from 1 to 5 mg/mL, reaching approximately 60 % at 5 mg/mL. Approximately 47% and 35% of rhEGF was released from the multivesicular liposomes within 6 h in simulated intra-gastric fluid (pH 1.2) and intra-intestinal fluid (pH 7.4), respectively. rhEGF-loaded multivesicular liposomes markedly suppressed the enzymatic degradation of the peptide in an incubation with the Caco-2 cell homogenate. However, the transport of rhEGF from the multivesicular liposomes to the basolateral side of Caco-2 cells was two times lower than that of the rhEGF in aqueous solution. The gastric ulcer healing effect of rhEGF-loaded multivesicular liposomes was significantly enhanced compared with that of rhEGF in aqueous solution; the healing effect of the liposomes was comparable to that of the cimetidine in rats. Collectively, these results indicate that rhEGF-loaded multivesicular liposomes may be used as a new strategy for the development of an oral delivery system in the treatment of peptic ulcer diseases.     

The targeting effect of H7K(R2)2-modified pH-sensitive liposomes on U87-MG cells

The present study aimed to investigate the targeting effect of H₇K(R₂)₂-modified pH-sensitive liposomes on U87-MG cells. Using coumarin-6 as a fluorescence probe, we prepared H₇K(R₂)₂-modified pH-sensitive liposomes (designated as coumarin-6-PSL-H₇K(R₂)₂). The flow cytometry assay was used to evaluate the effect of H₇K(R₂)₂ proportions on the cellular uptake and endocytosis pathways of coumarin-6-PSL-H₇K(R₂)₂ on U87-MG cells_. The circular dichroism (CD) spectroscopy assay was used to investigate the secondary structures of H₇K(R₂)₂ peptide at pH 7.4 and pH 6.8, respectively. Our results indicated that the 2.5% proportion of H₇K(R₂)₂ in the coumarin-6-PSL-H₇K(R₂)₂ was superior to those of 1% and 3.5% of H₇K(R₂)₂. The uptake of coumarin-6-PSL-H₇K(R₂)₂ on U87-MG cells was not inhibited by filipin, M-β-CD or chlorpromazine. The secondary structure of H₇K(R₂)₂at pH 6.8 was mostly presented as β-turn. In conclusion, we suggested that the appropriate proportion of H₇K(R₂)₂ in the H₇K(R₂)₂-modified pH-sensitive liposomes could be set at 2.5%. The cellular uptake pathway for H₇K(R₂)₂-modified pH-sensitive liposomes was via the cell penetrating capacity of H₇K(R₂)₂ which responded to acidic condition. The secondary structure of H₇K(R₂)₂ at pH 6.8, which was presented as the shape of hairpin, might be mainly responsible for its targeting and cell penetrating effect.     

Preparation and optimisation of anionic liposomes for delivery of small peptides and cDNA to human corneal epithelial cells

Drug delivery to corneal epithelial cells is challenging due to the intrinsic mechanisms that protect the eye. Here, we report a novel liposomal formulation to encapsulate and deliver a short sequence peptide into human corneal epithelial cells (hTCEpi). Using a mixture of Phosphatidylcholine/Caproylamine/Dioleoylphosphatidylethanolamine (PC/CAP/DOPE), we encapsulated a fluorescent peptide, resulting in anionic liposomes with an average size of 138.8?±?34?nm and a charge of ?18.2?±?1.3?mV. After 2?h incubation with the peptide-encapsulated liposomes, 66% of corneal epithelial (hTCEpi) cells internalised the FITC-labelled peptide, demonstrating the ability of this formulation to effectively deliver peptide to hTCEpi cells. Additionally, lipoplexes (liposomes complexed with plasmid DNA) were also able to transfect hTCEpi cells, albeit at a modest level (8% of the cells). Here, we describe this novel anionic liposomal formulation intended to enhance the delivery of small cargo molecules in situ.     

Mechanisms of drug release in pH-sensitive micelles for tumour targeted drug delivery system: A review

During the past decades, chemotherapy has been regarded as the most effective method for tumor therapy, but still faces significant challenges, such as poor tumor selectivity and multidrug resistance. The development of targeted drug delivery systems brings certain dramatic advantages for reducing the side effects and improving the therapeutic efficacy. Coupling a specific stimuli-triggered drug release mechanism with these delivery systems is one of the most prevalent approaches for targeted therapy. Among these approaches, pH-sensitive micelles are regarded as the most general strategy with advantages of increasing solubility of water-insoluble drugs, pH-sensitive release, high drug loading, etc.This review will focus on the potential of pH-sensitive micelles in tumor therapy, analyze four types of drug-loaded micelles and mechanisms of drug release and give an exhaustive collection of recent investigations. Sufficient understanding of these mechanisms will help us to design more efficient pH-sensitive drug delivery system to address the challenges encountered in targeted drug delivery systems for tumor therapy.     

表面活性剂-醇质体增强多烯紫杉醇经皮给药的渗透性研究(英文)

皮肤的屏障作用使大部分药物无法实现透皮给药。本文以改善难溶性大分子模型药物多烯紫杉醇(docetaxel,DTX)的经皮渗透性为主体思路,研制了DTX的表面活性剂-醇质体(surfactant-ethanlic liposomes,SEL)。SEL由磷脂、乙醇、胆酸钠、DTX和磷酸盐缓冲液组成,采用薄膜分散法制备。对SEL的囊泡形态(冷冻蚀刻电镜法)、粒径大小及分布进行了表征,并测定包封率和载药量。采用体外扩散池实验研究了DTX表面活性剂-醇质体的经皮渗透性。结果表明,当磷脂与表面活性剂的比例为85:15时,DTX的稳态透皮速率和累计透皮量均为最高,且优于表面活性剂脂质体、醇质体和普通脂质体。最优处方的粒径分布、形态和载药量均较为稳定。本研究表明,通过将DTX包载于SEL中可显著改善DTX的经皮渗透性。     

Localized delivery of paclitaxel using elastic liposomes: Formulation development and evaluation

In the present study an elastic liposomes-based paclitaxel formulation was developed with the objective to remove Cremophor EL. Cremophor EL is currently used for solubilizing paclitaxel in the marketed formulation and is known to produce toxic effects. Elastic liposomal paclitaxel formulation was extensively characterized in vitro, ex-vivo, and in vivo. The results obtained were compared against the marketed paclitaxel formulation. The maximum amount of paclitaxel loaded in the elastic liposomal formulation was found to be 6.0?mg/ml, which is similar to the commercial strength of marketed paclitaxel formulation. In vitro skin permeation and deposition studies showed 10.8-fold enhanced steady state transdermal flux and 15.0-fold enhanced drug deposition in comparison to drug solution. These results further confirmed with the vesicle–skin interaction study using FTIR technique. Results of the hemolytic toxicity assay indicate that elastic liposomal formulation induced only 11.2?±?0.2% hemolysis in comparison to the commercial formulation which showed 38?±?3.0%. Further, results of the Draize test showed no skin irritation of paclitaxel elastic liposomal formulation. Findings of the study demonstrate that elastic liposomes as a carrier is an attractive approach for localized delivery of paclitaxel.     

月桂酸二乙醇酰胺修饰利福喷丁脂质体的制备、性质及肺部给药

目的制备表面活性剂修饰利福喷丁(RIF)脂质体，进行该脂质体水化性能、载药量、释药速度和肺部给药研究。方法采用薄膜超声法制备利福喷丁脂质体，比较月桂酸二乙醇酰胺(LDEA)，Tween 80和azone修饰利福喷丁脂质体的形态、包封率、释药速度和离体猪肺膜透过性，通过纤支镜进行肺部给药研究。结果RIF-LDEA脂质体粒径在15～50 nm，包封率为83.0%，表观透膜系数Kp为44.29；LD₅₀为675 mg·kg^-1。结论LDEA修饰使利福喷丁脂质体的载药量增加1倍、释药速度的可调性强及安全性好。经纤支镜介导灌注给药治疗肺内膜结核的效果显著。     

A new vaginal delivery system of amphotericin B: a dispersion of cationic liposomes in a thermosensitive gel

Amphotericin B (AmB) is used in the treatment of fungal infections; however, its clinical use is limited by its toxic side effects. In this study, AmB-loaded cationic liposome gels were formulated with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), and cholesterol (CH) at a molar ratio of DOPE:DOTAP:CH?=?4:5:1 in thermosensitive gel composed of poloxamer 407 (P407) and poloxamer 188 (P188). To enhance the solubility of AmB, 6 mol% of distearoyl phosphatidyl ethanolamine–polyethylene glycol was added prior to encapsulation of the drug into liposomes. Scanning electron microscopy was used to observe the AmB encapsulated cationic liposome gels. In vitro release, stability and cytotoxicity of AmB in cationic liposome gels were evaluated. The particle size and zeta potential of AmB-loaded liposomes were in the range of 400–500?nm and 40–60 mV, respectively. The thermosensitive gel at the ratio of P407:P188?=?15:15 (w/w) gelled at 37°C, approximating body temperature. Encapsulation efficiency of AmB was ～50–60%, which was influenced by the ratio of AmB to lipid. Moreover, AmB-loaded cationic liposome gels were more stable and less toxic than free AmB. From these results, cationic liposome gel formulations may be useful for vaginal delivery of AmB.     

Cell-penetrating TAT peptide in drug delivery systems: Proteolytic stability requirements

The stability and activity of the HIV cell-penetrating TAT peptide (TATp) on the surface of TATp-modified micelles and liposomes in relation to its proteolytic cleavage was investigated. TATp moieties were attached to the surface of these nanocarriers using TATp modified with a conjugate of phosphatidyl ethanolamine with a ‘short’ PEG (PEG-PE). Following pre-incubation with trypsin, elastase, or collagenase, the proteolytic stability of TATp on the surface of these modified carriers was studied by HPLC with fluorescence detection using fluorenylmethyl chloroformate (FMOC) labeling. All tested enzymes produced a dose-dependent cleavage of TATp as shown by the presence of TATp Arg-Arg fragments. Inhibition of TATp cleavage occurred when these TATp-micelles were modified by the addition of longer PEG-PE blocks, indicating an effective shielding of TATp from proteolysis by these blocks. TATp-modified carriers were also tested for their ability to accumulate in EL-4, HeLa, and B16-F10 cells. Trypsin treatment of TATp-modified liposomes and micelles resulted in decreased uptake and cell interaction, as measured by fluorescence microscopy and fluorescence-activated cell sorting techniques. Furthermore, a decrease in the cytotoxicity of TATp-modified liposomes loaded with doxorubicin (Doxil) was observed following trypsin treatment. In conclusion, steric shielding of TATp is essential to ensure its in vivo therapeutic function.     

Dual-ligand modified liposomes provide effective local targeted delivery of lung-cancer drug by antibody and tumor lineage-homing cell-penetrating peptide

The abilities of a drug delivery system to target and penetrate tumor masses are key factors in determining the system’s chemotherapeutic efficacy. Here, we explored the utility of an anti-carbonic anhydrase IX (anti-CA IX) antibody and CPP33 dual-ligand modified triptolide-loaded liposomes (dl-TPL-lip) to simultaneously enhance the tumor-specific targeting and increase tumor cell penetration of TPL. In vitro, the dl-TPL-lip increased the cytotoxicity of TPL in CA IX-positive lung cancer cells, which showed tunable size (137.6?±?0.8?nm), high-encapsulation efficiency (86.3?±?2.6%) and sustained release. Dl-TPL-lip significantly improved the ability of liposomes to penetrate 3?D tumor spheroids and exhibited a superior inhibiting effect. Furthermore, pharmacokinetic studies in rats that received TPL liposomal formulations by endotracheal administration showed a reduced concentration of TPL (17.3%–30.6% compared to free TPL) in systemic circulation. After pulmonary administration in orthotopic lung tumor-bearing mice, dl-TPL-lip significantly enhanced TPL anti-cancer efficacy without apparent systemic toxicity. This dual-ligand modified liposomal vehicle presents a potential system for localized and targeted delivery of anti-cancer drugs to improve their efficacy.     

Design and characterization of liposomes containing long-chain N-acylPEs for brain delivery: penetration of liposomes incorporating GM1 into the rat brain

Purpose. To develop a suitable liposomal carrier to encapsulate neu- roactive compounds that are stable enough to carry them to the brain across the blood-brain barrier with the appropriate surface characteristics for an effective targeting and for an active membrane transport. Methods. Liposomes containing glycosides and a fusogenic lipid were prepared by extrusion. Photon correlation spectroscopy, fluorescence spectroscopy, and differential scanning calorimetry were used to characterize liposomal preparations. Tissue distribution was determined by using ³H-cholesterylhexadecylether as a marker. Results. The incorporation of glycoside determinants and N-palmitoylphosphatidylethanolamine gives liposomes with similar initial size, trapped volume, negative surface charge, bilayer fluidity, and melting temperature, except for monosialoganglioside-containing liposomes, which showed less negative surface charge and the highest size, trapped volume and melting temperature. All glycosilated formulations gave liposomes able to retain up to the 95% of encapsulated carboxyfluorescein after 90 min at physiologic temperature even in the presence of serum. Monosialoganglioside liposomes were recovered in the cortex, basal ganglia, and mesencephalon of both brain hemispheres. The liver uptake was higher for sulfatide- and glucose-liposomes, whereas the higher blood levels were observed for glucose- and mannose-liposomes. Conclusions. These results show the suitability of such liposomal formulations to hold encapsulated drugs. Moreover, the brain uptake of monosialoganglioside liposomes makes them good candidates as drug delivery systems to the brain.     

Taguchi正交法优化奥沙利铂长循环脂质体的制备

目的通过优化奥沙利铂(L-OHP)长循环脂质体的制备工艺,以提高其包封率(entrapment efficiency,EE%)并达到缓释效果。方法采用Taguchi正交法L9(34)(TOA),以包封率作为考察指标并采用超滤法检测包封率。考察了L-OHP脂质体的粒径、zeta电位、形态以及体外释放行为。结果药物与脂质间的比例(W/W),胆固醇与磷脂酰胆碱的比值(M/M)和超声时间是影响最大的三个因素(P<0.05)。此外,优化后的奥沙利铂脂质体经mPEG修饰后与未修饰的奥沙利铂脂质体相比较,具有更好的物理化学特性,其粒径为(204±1.1)nm,包封率高达25.40%±2.5%。修饰后的脂质体在透射电子显微镜下呈现球形结构,具有较大的内部空间,脂质体表面可见白色mPEG层状结构。经mPEG修饰的奥沙利铂脂质体相比于未修饰的脂质体,具有更低的体外释放速率,具有一定缓释能力。修饰与未修饰的脂质体以及奥沙利铂溶液均符合一级释放模型,拟合系数为0.990 2。结论经Taguchi正交法优化后的奥沙利铂长循环脂质体可作为一种新型的肿瘤靶向药物传递系统。     

Transdermal delivery of low molecular weight heparin loaded in flexible liposomes with bioavailability enhancement: comparison with ethosomes

Low molecular weight heparin (LMWH)-loaded flexible liposomes (flexosomes) were formulated for transdermal delivery, and their physicochemical and pharmacokinetic parameters were compared with LMWH-loaded ethosomes. Flexosomes had similar particle size compared with ethosomes, but their deformability was higher than that of ethosomes (76.7% vs. 46.8%). In vitro, flexosomes demonstrated 2.6-fold higher permeability coefficient than ethosomes. In comparison to LMWH aqueous solution, skin deposition of flexosome increased 3.2-fold, while that of ethosome increased only 2.0-fold. In vivo, after the topical application of flexosome to hairless mouse, [anti-Xa]_max was 1.11?IU/mL, while ethosomes showed only 0.32?IU/mL. Moreover, AUC_0–24?h of flexosomes was 2.5-fold higher than ethosomes. In conclusion, the enhanced skin permeation and bioavailability of LMWH can be achieved with flexosomes in comparison with ethosomes. The LMWH transdermal delivery via flexosomes has the potential to replace the parenteral dosage forms for the treatment of venous thromboembolism, pulmonary embolism and cardiovascular events.